Ok. So, the thermostat is already covered. If you're not experiencing any mysterious coolant loss or signs of water in the oil, then I'd suspect a possible problem with flow through the radiator... either coolant flow or air flow. Excessive dirt, mud, ect in the fins or the typical crud that builds up in the flues over time & miles can reduce the radiator's cooling capacity, and this will usually show up at highway speeds or under load.
I don't see any reason to suspect the fan clutch at this point because fan clutch problems will usually show themselves at idle or in slow traffic.